This invention relates broadly to the field of respiratory therapy equipment. More specifically, it relates to a system for blending breathable gases (e.g., air and oxygen) for delivery to a patient that is breathing with the assistance of a ventilator.
Patients that are breathing with the assistance of a ventilator often require the mixture of air with another gas. For example, many mechanically-ventilated patients require oxygen-enriched air. Consequently, many ventilators incorporate a mechanism or system for blending air and oxygen in adjustable proportions. See, for example, U.S. Pat. No. 4,072,148 to Munson et al.; U.S. Pat. No. 3,768,468 to Cox; U.S. Pat. No. 4,340,044 to Levy et al.; and U.S. Pat. No. 3,916,889 to Russell.
Recently, efforts to control medical costs have led to increasing use of so-called "home care" ventilators. Such ventilators are compact enough to be transportable, allowing many patients who are not critically ill to continue their respiratory therapy in their homes. These home care ventilators typically employ a reciprocating piston pump to deliver air to the patient in accordance with a prescribed flow rate-versus-time curve that is a function of several adjustable parameters, such as minute volume, respiratory rate, and inspiratory-to-expiratory time ratio, for example. While these relatively compact ventilators have many of the features of the larger, "critical care" ventilators used in hospitals, the requirement of transportability does impose some limitations. One such limitation, for example, has been the lack of a convenient means for oxygen has been the lack enrichment that would provide accurate control of the concentration of oxygen, and which would allow the concentration to remain substantially constant throughout the inspiratory stroke of the piston.
In the prior art, for example, oxygen is typically added to the delivered air flow downstream from the ventilator piston. The oxygen is typically supplied from a compressed gas tank or a flow-controlled source, such as an oxygen concentrator. There are, however, problems with this approach.
First, there is a problem with adequate mixing of the air and oxygen when the oxygen is added to the gas flow somewhere between the pump and the patient. Another important problem is the difficulty in maintaining a relatively constant percentage of oxygen throughout the inspiratory stroke of the piston. This latter problem has two sources: (1) the oxygen is supplied at a substantially constant flow, while the air flow from the piston is time-variable; and (2) the oxygen flow rate must be carefully readjusted whenever the flow parameters of the ventilator are changed.
The prior art has attempted to overcome these problems through the use of accumulators and labyrinthine flow paths to achieve gas mixing. Such mechanisms do little to address the constant proportion problem, and they "solve" the mixing problem at the expense of accuracy and repeatability. Moreover, they tend to "dump" excess oxygen to the atmosphere, which is not only wasteful, but which also poses a potential fire hazard.
Prior art gas blending mechanisms exist that address the problem of providing a constant blend proportion in the face of varying flow rates demanded by the ventilator. See, for example, U.S. Pat. No. 4,072,148 to Munson et al. Such mechanisms, however, are adapted for applications, such as in critical care ventilators, where the constituent gases are both supplied from pressure-regulated (and, preferably, pressure-balanced) sources. In home care ventilators, by contrast, the air is supplied to the ventilator from the ambient atmosphere, while the oxygen is supplied from a pressurized source. Therefore, blending mechanisms that need both gas sources pressure-regulated do not work in home care ventilators.
Still another problem associated with oxygen enrichment in a home care ventilator is that of oxygen wastage. Such ventilators require the use of relatively small oxygen cylinders, lest the purpose of transportability be defeated. This requirement militates against the use of oxygen dilution devices, of the type disclosed in U.S. Pat. No. 4,036,253 to Fegan et al., which blend air and oxygen by diluting a more or less constant flow of oxygen with an adjustable flow of air.
Accordingly, there has been a need for a gas blending system that is particularly adapted to home care ventilators of the piston pump type. Specifically, the need has been felt for a gas blending system that can be employed upstream of the pump to assure adequate gas mixing without wasting oxygen, and that accurately provides substantially constant proportions of the blended gases throughout the intake stroke of the piston pump. Such a system should also provide for convenient and accurate adjustment of the selected gas proportions. Furthermore, in the context of oxygen enrichment for a home care ventilator, such a blending system should minimize wasteful oxygen usage. These features should, advantageously, be provided in a system that is relatively simple to operate and maintain, and that can be conveniently employed as an "add-on" option with a wide variety of ventilators.